Traditionally, self-contained underwater breathing apparatuses can be viewed as falling into two general categories; open circuit and closed or semi-closed circuit. Open circuit systems are typically recognized by the common term SCUBA and represent the most commonly used form of underwater breathing apparatus. Developed and popularized by Jacques Cousteau, open circuit scuba apparatus generally comprises a high pressure tank filled with compressed air, the tank coupled to a demand regulator which supplies the breathing gas to for example, a diver, at the diver's ambient pressure, thereby allowing the user to breathe the gas with relative ease.
Conventional open circuit self contained diving systems are very well understood in the art and have been developed over the past several years into a wide variety of gas delivery systems, configured for an equally wide variety of applications. For example, compressed air is used as a breathing gas in typical sport diving applications, while one or more artificial mixtures of gasses might comprise the breathing mixture for diving operations at depths greater than approximately 50 meters (150 feet).
While open circuit scuba apparatus is relatively simple, at least in its compressed air form, the equipment required is bulky, heavy and the design itself is inherently inefficient in its use of the breathing gas. Each exhaled breath is expelled to the surrounding environment, thus wasting all the oxygen which was not absorbed by the user during the breath. This inefficiency in breathing gas utilization normally requires a diver to carry a large volume of breathing gas, in order to obtain a reasonable dive time. For example, conventional open circuit scuba gear typically includes compressed air tanks having gas volumes of about 80 cubic feet, and which weigh over 40 lbs.
As a diver descends, the ambient pressure increases approximately one atmosphere for every 30 feet of depth as is well known. Accordingly, gas consumption increases rapidly with depth. As a diver proceeds below approximately 150 feet, the increasing ambient pressure and thus the increasing pressure of the breathing gas, causes serious physiological problems, such as nitrogen narcosis and oxygen toxicity, which may have even deadly effects.
In addition, even short duration dives at depths greater than 100 feet require a certain amount of decompression time which must be pre-calculated in order to ensure a sufficient volume of breathing gas remains after the dive in order to accommodate decompression. Accordingly, while relatively simple and inexpensive, open circuit scuba apparatus imposes a number of practical limitations on both depth and dive time as a consequence of its construction and configuration.
The most common type of open circuit SCUBA apparatus is depicted in FIG. 1 and is of the open circuit demand-type which utilizes compressed air tanks in combination with demand regulator valves which provide air from the tanks on demand from a diver 18 by the inhalation of air. A compressed air supply tank 10 is coupled to a first stage (high pressure) regulator 12 which conventionally including an on-off valve 11 which reduces the pressure of the air within the tank to a generally uniform low-pressure value suitable for use by the rest of the system. Low pressure air (approximately 150 psi) is delivered to a second stage regulator 14 through a demand valve 16 in conventional fashion. Compressed air, at the cylinder pressure, is reduced to the diver's ambient pressure in two stages, with the first stage reducing the pressure below the tank pressure, but above the ambient water pressure, and the second stage reducing the gas pressure to the surrounding ambient or water pressure. The demand valve is typically a diaphragm actuated, lever operated spring-loaded poppet which functions as a one-way valve, opening in the direction of air flow, upon movement of the diaphragm by a diver's inhalation of a breath.
The second form of self contained breathing apparatus is the closed circuit or semi-closed circuit breathing apparatus, commonly termed rebreathers. As the name implies, a rebreather allows a diver to "rebreathe" exhaled gas to thus make nearly total use of the oxygen content in its most efficient form. Since only a small portion of the oxygen a person inhales on each breath is actually used by the body, most of this oxygen is exhaled, along with virtually all of the inert gas content such as nitrogen and a small amount of carbon dioxide which is generated by the diver. Rebreather systems make nearly total use of the oxygen content of the supply gas by removing the generated carbon dioxide and by replenishing the oxygen content of the system to make up for that amount consumed by a diver.
Both types of rebreather systems mentioned above, comprise a certain few essential components; namely, a flow loop with valves to control the flow direction, a counterlung or breathing bag, a scrubber to absorb or remove exhaled CO.sub.2, and some means to add gas to the counterlung as the ambient pressure increases. Valves maintain gas flow within the flow loop in a constant direction and a diver's lungs provides the motive power.
A typical semi-closed circuit rebreather system is illustrated in FIG. 2 and commonly comprises a compressed gas cylinder 20 conventionally including an on-off value 11 and first stage, high-pressure regulator 12, containing a specific gas mix having a predetermined fraction of oxygen. The gas is provided to a flow loop 22, generally implemented by flexible, gas impermeable hoses, which are coupled between the cylinder 20 and a flexible breathing bag 24, sometimes termed a counterlung. A pair of one-way check valves 26 and 28 are disposed in the flow loop such that the gas flow within the loop is maintained in a single direction (clockwise in the illustration of FIG. 2). An exhaled breath would thus enter the counterlung, increasing the pressure therein, and pass through one-way check valve 26 and move through some device means to remove excess carbon dioxide from the breathing gas, such as a CO.sub.2 canister 30, and thereby return to the counterlung through one-way check valve 28. The check valves thus maintain the gas flow in a constant direction, while the diver's lungs move the gas through the CO.sub.2 canister in the system. The gas mix is introduced into the flow loop at a flow rate calculated to maintain the oxygen needs of a particular diver during the dive. Gas is introduced to the flow loop at a constant fixed flow rate through a valve 32 coupled between the flow loop and the first stage regulator 12 of the gas cylinder 20. As the breathing gas mix is recirculated, some of the oxygen is necessarily consumed and CO.sub.2 is absorbed, thus perturbing both the total volume and the mix of the gas. A portion of the oxygen is consumed during recirculation, so the diver necessarily breathes a mixture with a lower oxygen concentration than that of the gas mix. Since the amount of oxygen supplied to the system depends on a diver's activity level (oxygen consumption rate), care must be taken to take activity into account as well as selecting the gas mixture composition for a particular diving depth.
A more efficient type of rebreather system is the closed circuit rebreather, illustrated in simplified form in FIG. 3. Closed circuit rebreathers are generally more sophisticated and effective in their maintenance of oxygen levels in the flow loop. Nonetheless, they share common components with semi-closed circuit rebreather systems such as that depicted in FIG. 2. The main contrast between fully closed and semi-closed circuit rebreather systems is that the closed circuit rebreather, as configured, provides a source of pure oxygen to the flow loop and introduces oxygen to the recirculating gas in an amount ideally equal only to that consumed by a diver such that system mass is conserved. The oxygen level (more correctly the oxygen partial pressure) is monitored electronically by an oxygen sensor (34 in FIG. 3) whose output is evaluated by a processing circuit (36 of FIG. 3) which, in turn, controls an electrically operated solenoid valve so as to add oxygen to the system when the oxygen sensor indicates it is being depleted. It should be noted, that closed circuit rebreathers only introduce gas to the system when the oxygen sensor 34 indicates the need for additional oxygen or as ambient pressure increases during descent and the addition of diluent is required to prevent the collapse of the counterlung. Oxygen is added in "pulses" in contrast to the steady-state flow of the semi-closed circuit system and is required to be constantly monitored. Diluent from an optional diluent gas source (indicated in phantom in FIG. 3) is added by a demand valve in the counterlung that is activated as the counterlung collapses because of increasing ambient pressure.
It should likewise be noted that once a particular oxygen partial pressure has been established in a closed circuit rebreather system, this partial pressure of oxygen is maintained by operation of the oxygen sensor 34 and processing circuit 36, regardless of a diver's external environment, and any changes thereto.
Partial pressure of oxygen in a particular breathing gas mixture may be understood as the pressure that oxygen alone would have if the other gasses (such as nitrogen) were absent from the gas. The physiological effects of oxygen depend upon this partial pressure in the mix and serious consequences result from oxygen partial pressures that are too high; e.g., oxygen becomes increasingly toxic as the partial pressure increases significantly above the oxygen partial pressure found in air at sea level (0.21 atmospheres), as well as too low. Where the oxygen partial pressure is too low, a diver would not necessarily experience any discomfort or shortness of breath, and in many cases may not even be aware of the shortness of oxygen until unconsciousness is imminent. In a relatively short period of time, depending in turn on the volume of a counterlung, the diver would become unconscious and eventually die from hypoxia The diver would experience very little discomfort, and in fact may feel rather euphoric. This euphoria is a typical and characteristically dangerous aspect of hypoxia.
On the other hand, serious physiological effects may result from too much oxygen leading to various forms of what might be termed oxygen poisoning. There are several major forms of oxygen poisoning but two in particular have a bearing on the operational configuration of various rebreather systems; central nervous system toxicity (CNS) and pulmonary or whole-body oxygen poisoning. Almost any rebreather system that includes an oxygen supply component is capable of delivering excess oxygen to a diver. Excess oxygen is defined in this case as oxygen partial pressure greater than specific tolerable limits; the most important limit being that of CNS oxygen toxicity. CNS limits, which define the oxygen partial pressure levels that can be tolerated for various durations depending on the degree of oxygen excess, are defined in the 1991 National Oceanographic and Atmospheric Administration (NOAA) diving manual and are well understood by those skilled in the art. CNS poisoning becomes a significant consideration as the partial pressure of oxygen exceeds a generally accepted limit of 1.6 atmospheres. CNS toxicity gives rise to various symptoms, the most serious of which are convulsive seizures, similar to those experienced during an epileptic fit. These seizures generally last for about 2 minutes and are followed by a period of unconsciousness.
If a level of 1.6 atmospheres is not exceeded, then the concern becomes one of pulmonary or whole body toxicity rather than CNS. Pulmonary oxygen toxicity results from prolonged exposure to oxygen partial pressures above approximately 0.5 atmospheres and the consequences of excessive exposure include lung irritation, which may be reversible, and some lung damage which is not.
It will be apparent from the foregoing, that the partial pressure of oxygen in a breathing gas mixture should be kept to a value in the range of from about 0.21 atmospheres to about 1.6 atmospheres. Further, in the absence of pulmonary oxygen toxicity considerations, the optimum choice of the partial pressure of oxygen is the maximum value for which CNS toxicity poses no threat, i.e., 1.6 atmospheres. This is because maximizing the oxygen partial pressure to the highest practical limit has the effect of minimizing the diluent partial pressure and, minimizing diluent physiological uptake which leads to the need for decompression. Accordingly, to the extent that oxygen partial pressure is increased, decompression times are correspondingly decreased. However, for long duration dives or multiple repetitive dives, pulmonary oxygen toxicity (rather than CNS) presents additional limitations that could be avoided by a choice of a lower partial pressure of oxygen. This choice depends on well known pulmonary toxicity limitations, breathing gas tank capacity, and decompression considerations.
Thus, it will be seen that there is no one specific partial pressure of oxygen in a breathing gas that is optimal for all conditions at all depths. One set of factors would tend to indicate that a relatively higher partial pressure of oxygen is preferred, while another set of factors would tend to indicate that this is not always the case.
Typical of prior art systems is a mixed-gas, closed circuit rebreather disclosed in U.S. Pat. No. 4,939,647 to Clough et al. The Clough et al. system is based on a conventional Rexnord CCR 155-type closed circuit rebreather comprising a supply of compressed inert gas and a supply of oxygen in separate source bottles. Inert gas is fed into the system's breathing loop by a demand regulator in order to maintain a loop volume with increasing depth, while oxygen is added to the breathing loop as it is consumed by a diver. Oxygen partial pressure in the loop is electronically monitored and maintained to a pre-set level below the CNS threshold. The system includes three oxygen sensors, operating in a majority-vote configuration which provides the sensing function for determining oxygen partial pressure within the loop. Oxygen partial pressures are adjustable, depending on the dive profile chosen, but once a particular value has been pre-set, that value is maintained unless affirmatively readjusted. As a result, the Clough et al. system results in unnecessary restrictions in a dive profile.
Similar rebreather systems are described in U.S. Pat. No. 3,727,626 to Kanwisher et al. and U.S. Pat. No. 4,236,546 to Manley et al. The systems described are both closed circuit-type rebreathers that include electronics for maintaining oxygen partial pressures in a breathing loop at a specific, pre-set value.
The net result of a pre-set value of PO.sub.2 can result in a reduction of dive time and an increase in unproductive decompression times. The objective of the present invention is to prevent these limitations.